This invention relates to a fractionation of blood cell-containing liquid suspensions and, more particularly, to an apparatus for effecting such fractionation by filtration through a microporous membrane.
Certain highly desirable blood processing procedures require the ability to effect an efficient separation of a liquid suspension of blood cellular components into a cellular component-containing fraction and a cellular component-free liquid fraction without causing damage to the cellular components. For example, the preservation of red blood cells, white blood cells or platelets which have been separated from whole blood for future use in transfusions, can be effectively achieved by freezing a prepared suspension of the blood cells in an electrolyte solution containing a suitable concentration of a cryoprotective agent, such as glycerol or dimethyl sulfoxide. Since the concentration of the cryoprotective agent required for the freezing procedure is well above physiologically tolerable levels, the prepared blood cell suspension must be fractionated subsequent to thawing and prior to use so as to remove the cryoprotective agent therefrom or at least to reduce its concentration in the suspension to a physiologically tolerable level. Two techniques are currently available for effecting such fractionation, one based upon the reversible agglomeration of blood cells in the presence of carbohydrates, and the other upon various centrifugation procedures.
The problems associated with the removal of cryoprotective agents has been one of the major obstacles standing in the way of more extensive clinical use of frozen cells.
In the field of red cell freezing, various advantages have been cited for promoting the use of this product. They include a possible reduction in hepatitis transmission, a reduction in transmission of undesirable antigens and antibodies, and most important, a prolonged storage period permitting accumulation of "rare red cells" blood for autologous transfusion, and stockpiling for use during shortages. Current technology can be used to achieve these goals but a more simple and efficient system is needed.
Platelets frozen storage is desirable in order to reduce outdating and allow for provisions of "matched" or autologous cells. Techniques currently in use are not satisfactory and the microporous system may be suitable for such an application. Similarly, white cell storage is a problem and transfusion of unfrozen products are still basically experimental. However, it is expected that utilization will increase, and that frozen storage will be needed for their efficient management.
Another highly desirable blood processing procedure involving the separation of a liquid suspension of blood cellular components into a cellular component-containing fraction and a cellular component-free liquid fraction, is plasmapheresis. Plasmapheresis is defined as the process of removal of whole blood from the body of a blood donor by venesection, separation of its plasma portion, and reintroduction of the cellular portion into the donor's bloodstream. The cell-free plasma thus collected may either be used directly for patient care or further processed into specific plasma derivatives for clinical use. The return of the cellular components to the donor provides this plasma collection procedure with the advantage that it enables donations by the donor at more frequent intervals. In addition to its use for plasma collection, plasmapheresis also has therapeutic implications in plasma exchange procedures for the treatment of various clinical disorders.
Currently, the most efficient and commonly employed techniques for carrying out the plasmapheresis procedure utilize "batch" centrifugation systems for effecting the separation of the cell-free plasma from the whole blood. The most serious drawback with these currently used techniques is the relatively long period of donor time which they require, typically ranging from one to one and a half hours or more for collecting 500 ml of cell-free plasma. Such long period of donor time tends to have a detrimental effect upon the recruitment of volunteer donors and upon the overall cost-effectiveness of the plasmapheresis procedure.
Techniques for the separation of cell-free plasma from whole blood by filtration through a microporous membrane have previously been proposed. For example, in U.S. Pat. No. 3,705,100, issued Dec. 5, 1972, to Blatt et al, there is disclosed a blood fractionating process and apparatus wherein whole blood is conducted in laminar flow across the surface of a microporous membrane along a flow path which is substantially parallel to the upstream side of the membrane under pressure conditions at the inlet and outlet ends of the flow path sufficient to maintain the laminar flow and to provide a filtration driving force from the upstream side to the downstream side of the membrane. Cell-free plasma is recovered as filtrate from the downstream side of the membrane, and the cellular component-containing fraction is recovered from the outlet end of the flow path. The patent teaches that one embodiment of the process and apparatus disclosed therein is capable of separating approximately 3.0 to 3.4 ml of plasma from a 10 ml sample of fresh blood of normal hematocrit in a filtering time of 15 to 20 minutes. While such filtering capacity may be adequate for the in vitro processing of relatively small amounts of plasma for subsequent physical, chemical or clinical analyses, it obviously would not be sufficient for practical utility in plasmapheresis, where the objective is to collect 500 ml of cell-free plasma in certainly no greater and preferably substantially less than the 60 to 90 minutes required by the standard plasmapheresis techniques.
In attempting to scale up the filtration process and apparatus disclosed in the Blatt et al patent to a filtration capacity sufficient for practical utility in carrying out the plasmapheresis procedure, a number of interrelated factors must be taken into consideration. First of all, in order to minimize the total required membrane area so that the resulting filtration module will be reasonably compact in size, and in order to minimize the required period of donor time, it is most desirable to operate under conditions which will provide optimal filtrate flux, i.e., filtration rate per area of membrane. Since, in certain cases, the filtrate flux will be governed primarily by the transmembrane pressure, i.e., the pressure differential between the upstream and downstream sides of the membrane providing the filtration driving force, the transmembrane pressure should be maintained sufficiently high so as to maximize the filtrate flux. However, too high a transmembrane pressure will cause the blood cellular components to be forced to the membrane surface and interact therewith, leading to irreversible damage or hemolysis of the cells or possibly even to plugging of the membrane pores. Proper control of the transmembrane pressure so as to provide optimal filtration rate per area of membrane without causing damage to the cellular components is further complicated by the pressure drop from the inlet end to the outlet end of the blood flow path, which causes corresponding variations in the transmembrane pressure through the system. A relatively high pressure drop could lead to a very low transmembrane pressure in the outlet region. Thus, in order to insure that the transmembrane pressure in the outlet region will be maintained sufficiently high for efficient operation, the transmembrane pressure in the inlet region must be correspondingly higher so as to compensate for the pressure drop through the system. Moreover, if the system is to be used for carrying out a truly continuous flow plasmapheresis procedure wherein the cellular component-containing fraction exiting from the outlet end of the filtration flow path is directly reinfused into the donor's bloodstream, a further factor influencing the transmembrane pressure through the system is the requirement that the pressure at the outlet end of the filtration flow path be at least sufficient to overcome the sum of the return venous blood pressure and the pressure drop in the return needle and tubing assembly is an accessory blood pump is to be avoided.
An improvement in the filtration process described in the aforementioned Blatt et al patent, is described and claimed in the copending U.S. patent application of Leonard I. Friedman, Franco Castino, Michael J. Lysaght and Barry A. Solomon, filed on even date herewith U.S. Ser. No. 909,458 , entitled "PROCESS FOR SEPARATING BLOOD CELL-CONTAINING LIQUID SUSPENSIONS BY FILTRATION", and incorporated herein by reference. This improvement consists of controlling the membrane wall shear rate of the suspension along the filtration flow path so that such shear rate will be sufficiently high to cause axial migration of cells and inhibit interactions of the cellular components with the membrane surface at the particular transmembrane pressure conditions employed and sufficiently low so as not to itself induce mechanical lysis or damage to the cellular components. It was found that by properly correlating the membrane wall shear rate with the particular set of transmembrane pressure conditions employed, it is possible to operate at transmembrane pressures providing optimal filtration rate per area of membrane while at the same time inhibiting lysis-causing interractions of the cellular components with the membrane surface which would otherwise occur at lower membrane wall shear rates. As disclosed in said copending Friedman, et al application, such improvement enables the filtration process to be scaled up to a filtration capacity rendering it practical for use as the blood separation technique in a continuous flow plasmapheresis system, requiring a substantially shorter period of donor time than that required by the standard centrifugal techniques conventionally employed for this purpose; and furthermore broadens the applicability of the filtration process to also render it a relatively simple, efficient and economical technique for effecting removal of cryoprotective agent from a previously frozen, thawed preparation of blood cells.
As disclosed in said copending Friedman et al application, the membrane wall shear rate of the blood cell-containing liquid suspension along the filtration flow path is a function of both the inlet suspension flow rate and the filtration flow channel dimensions, increasing with increasing flow rates and/or decreasing flow channel dimensions. Thus, once the operating membrane wall shear rate has been determined so as to properly correlated with the transmembrane pressure conditions being employed to provide optimal filtrate flux without damage to the cellular components, such shear rate can be achieved by proper coordination of the inlet suspension flow rate with the filtration flow channel dimensions.